Approximately 200,000 women will be diagnosed with breast cancer this year and more than 40,000 of those will die from the disease. About 25-30% of breast cancer patients overexpress the human epidermal growth factor receptor 2 (HER2/neu/ErbB2). HER2 has been linked to poor prognosis, high rate of metastasis, high risk of relapse, resistance to chemotherapy or hormone replacement therapies, and rapid progression to death. Ross J S, Fletcher J A, The Oncologist; 3:237-52 (1998). Trastuzumab (Herceptin) is a humanized monoclonal antibody that binds HER2 with high affinity. Passive immunotherapy with trastuzumab has dramatically improved outcomes for HER2-positive breast cancer patients. Dawood et al., J Clin Oncol.; 28:92-8A (2010). A limitation of immunotherapy with trastuzumab is the short half-life requiring frequent administration. Furthermore, passive immunotherapy with trastuzumab does not protect patients from development of metastasis or recurrence. To overcome the challenges of passive immunotherapy, cancer vaccines are under development and clinical testing. Disis et al., Immunology, 93:192-9 (1998); Nanda N K, Sercarz E E, Cell, 82:13-7 (1995).
HER2 cancer vaccines have several advantages compared to passive immunotherapy. Ladjemi et al., Cancer immunology, immunotherapy:CII, 59:1295-312 (2010). Establishment of a memory immune response could overcome resistance to passive immunotherapies upon repeated usage. A cancer vaccine holds the promise to prevent recurrence of the disease or progression to metastatic disease. Administration of a prophylactic vaccine (in high risk groups) has the potential to prevent the development of the disease before doctors would be able to diagnose its onset. Lastly, cancer vaccines offer practical advantages such as lower costs based on a less intensive treatment schedule.
While T cell and antibody-mediated immunity specific to HER2 exists in some patients, the majority of patients show immune self-tolerance to HER2 due to its fetal origin. Seliger B, Kiessling R. Trends in molecular medicine, 19:677-84 (2013). The HER2 protein, like most other tumor-associated antigens, represents an overexpressed or abnormally expressed gene product. Tolerance to such self-gene products is often mediated by many different mechanisms, with one being depletion of reactive high-avidity T cells against the antigen. T cell depletion through self-tolerance is not absolute, and reactive low-avidity T cells may be present; immunotherapeutic approaches are based on the ability to involve these low-avidity T cells in anti-cancer immunity though activation and expansion. Morgan et al., J Immunol., 160:643-51 (1998). HER2 immunogenicity is also impaired by abnormally low surface (major histocompatibility complex) MHC-I expression on tumor cells that limits or abolishes immune recognition by reactive cytotoxic T lymphocytes (CTLs). Immunotherapy outcomes could be improved by overcoming resistance arising from low MHC-I expression by approaches involving innate immunity mediated via antibody dependent cytotoxicity (ADCC) with natural killer (NK) cells and monocytes playing key roles. Musolino et al., J Clin Oncol, 26:1789-96 (2008). This mechanism is relevant to both passive antibody therapy (e.g. trastuzumab) and active vaccination approaches targeting humoral immunity. Triulzi et al., Cancer research, 70:7431-41 (2010).
Many different strategies have been proposed to overcome self-tolerance associated with the HER2 self-antigen, including depletion of regulatory T cells (Weiss et al., PLoS One. 2012; 7:e31962), altering the natural antigen to enhance immunogenicity, or presenting antigenic HER2 epitopes to the host in an altered molecular environment (foreign to the host). Disis et al., Journal of immunology, 156:3151-8 (1996). Approaches include vaccines based on proteins, peptides (Ladjemi et al., Cancer immunology, immunotherapy:CII. 2010; 59:1295-312), DNA (Radkevich-Brown et al., Cancer research. 2009; 69:212-8), anti-idiotype antibodies (de Cerio et al., Oncogene. 2007; 26:3594-602), autologous cells, dendritic cells (Saha A, Chatterjee S K., Cellular immunology. 2010; 263:9-21), and tumor cells. Dols et al., Journal of immunotherapy. 2003; 26:163-70.
Peptide-based vaccines constitute the largest group of cancer vaccines under preclinical and clinical evaluation. Several HER2 peptides derived from the extracellular domain (Mittendorf et al., Cancer immunology, immunotherapy:CII. 2008; 57:1511-21), transmembrane domain (Mittendorf et al., Cancer. 2006; 106:2309-17) or intracellular domains (Disis et al., Journal of clinical oncology, 2004; 22:1916-25) are in clinical trials as single-epitope or in combinations as multi-epitope vaccines. Several approaches have been shown to generate a HER2-specific response mediated by CTLs (cellular immunity) and/or humoral immunity. Dakappagari et al., Journal of immunology. 2003; 170:4242-53; Jasinska et al., Int J Cancer. 2003; 107:976-83. Nevertheless, peptide-based vaccines suffer from weak and short-lived immunogenicity and are dependent on adjuvants. In the absence of suitable adjuvants the peptides are prone to proteolytic degradation resulting in shorter circulation times. Thus, there is a need for improved vectors and epitope presentation strategies to develop stable peptide-based vaccines.
Antigen presentation systems (De Temmerman et al., Drug discovery today. 2011; 16:569-82; Bramwell V W, Perrie Y., Journal of Pharmacy and Pharmacology. 2006; 58:717-28), including virus-based platforms, emulsions, liposomes, as well as gel formulations, protect the antigen against proteolytic degradation, facilitate uptake by antigen-presenting cells (APCs) through passive or active targeting, and allow for co-delivery of antigens. Krishnamachari Y, Salem A K, Advanced drug delivery reviews, 61:205-17 (2009). Further, reports indicate that generation of tumor antigen-specific CTLs requires cross-priming of tumor antigens by APCs. Ridge et al., Nature, 393:474-8 (1998). Therefore, antigen delivery via virus-based platforms, which naturally interact with APCs thereby enhancing antigen delivery, may be an advantageous strategy for the development of cancer vaccines. While uptake of soluble antigen is primarily mediated by endocytosis, particulate vaccines are internalized through phagocytosis into phagosomes and thus are presented on MHC class II. Howland S W, Wittrup K D., Journal of immunology, 180:1576-83 (2008). Large quantities of antigens can be delivered, and a prolonged extracellular or intracellular release will foster prolonged antigen presentation by APCs. Shen et al., Immunology, 117:78-88 (2006).
Plant virus-based vectors displaying antigenic peptides fused to the coat proteins can be readily purified, and presentation of multiple copies of antigen on a macromolecular assembly can significantly enhance the immunogenicity of these epitopes. Jegerlehner et al., Vaccine, 20:3104-12 (2002). Several chimeric platforms have been shown to elicit protective immunity in diverse hosts in preclinical settings. Canizares et al., Immunology and cell biology, 83:263-70 (2005). PVX-based vaccine formulations have been developed and tested, for example: PVX-gp41 displaying HIV-1 epitopes (Marusic et al., Journal of Virology. 2001; 75:8434-9), PVX-R9 displaying hepatitis C virus (HCV) epitopes (Uhde-Holzem et al., Journal of virological methods. 2010; 166:12-20), PVX-Staphylococcus aureus D2 FnBP (Brennan et al., Vaccine, 1999; 17:1846-57), PVX-influenza-A virus nucleoprotein epitopes (Lico et al., Vaccine, 2009; 27:5069-76), and PVX-16E7 formulations displaying human papillomavirus (HPV) epitopes (Massa et al., Human gene therapy, 2008; 19:354-64). Immunization studies have shown that cellular and humoral immune responses can be triggered and epitope-specific antibodies were generated, demonstrating the utility of PVX as a presentation strategy.